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I turned on my furnace and went outside and noticed hot air blowing out a J-shaped PVC pipe. Is it a vent or error that needs to be plugged?

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So hot that you cant hold your hand over it?!?! It is going straight out into the atmosphere. That is what mine is doing... It doesn't seem correct. – user18509 Dec 9 '13 at 3:08
This isn't an answer, but to answer your question, all exhaust from a furnace is really hot. High-efficiency ones will just be 'less hot'. – DA01 Dec 9 '13 at 3:26
Oh sorry didn't mean to mark it as an answer... It is marked as HE on the side and it has an exhaust vent that is literally so hot that i can only stand to put my hand over it for about 5 seconds before it starts to hurt.. – user18509 Dec 9 '13 at 3:30
while that's hot, it's likely not nearly as hot as your standard exhaust flu temperatures, which can burn you almost instantly. – DA01 Dec 9 '13 at 3:33
So I am worrying over nothing?! Just seems to be such a waste of heat... – user18509 Dec 9 '13 at 3:34

It's a high efficiency gas furnace, and that's the exhaust vent. DO NOT cover or plug it!

A normal furnace relies on the stack effect, to carry hot exhaust up and out of the building. Since high efficiency furnaces extract most of the heat from the exhaust, the exhaust must be forced out. Since it takes less energy to move the cooled exhaust horizontally, HE furnaces typically exhaust to the side of a building rather than the top. As the gases are cooler, PVC, CPVC, or ABS piping can be used to carry the exhaust to the outside.

In a normal furnace you'll typically only see one motor and fan (blower), which is used to draw in cool air and blow warm air throughout the house. In an HE furnace you'll also find a draft inducer or purge motor and fan, which is used to draw in combustion air and push out exhaust.

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OP mentions hot air blowing out (and I see the same thing with mine). Does this indicate that heat extraction is less than optimal? – alt Oct 18 '13 at 19:12
@alt No. There will be some heat left in the exhaust, unless the furnace is 100% efficient. – Tester101 Oct 19 '13 at 12:15

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